[llvm] [CodeGen] Add laneBitmask as new MachineOperand type, utilised by newly defined COPY_LANEMASK instruction (PR #151944)

[Vikash Gupta via llvm-commits]

vg0204 wrote:

> Please post an RFC on discourse explaining what problem this solves and how.

I am in need of MO_Lanemask as well to encode liveness information for source registers as in case of COPY (or SPILL) instructions, that  can be used for selective expansion of those parts of registers tuple which are defined(#151123 & #151124).

So, should I still go with RFC explaining above point & the one given by Matt, along with its potential use-cases inf future.

https://github.com/llvm/llvm-project/pull/151944